1. Field of the Invention
The present invention relates to a base station selection method. More particularly, the present invention relates to a dynamic master base station selection method for clock synchronization among base stations in an in-building base station environment.
2. Description of the Related Art
It is necessary to establish synchronization among base stations in a mobile communication system, and typically an outdoor base station acquires synchronization with Global Positioning System (GPS) signals received from satellites. However, in an in-building environment, GPS signal reception is deteriorated which is an obstacle for installing an in-building base station.
In one proposed solution to address this problem, the conventional Institute of Electrical and Electronic Engineers (IEEE) 1588 standard determines a master base station and slave base stations among a plurality of in-building base stations and establishes synchronization of the slave base stations to the master base station. This method has a drawback in that, when the master base station fails to receive a GPS signal, all the slave base stations also fail to acquire synchronization.
Also, even when there is a base station that can provide a precise clock signal as compared to the current master base station, the slave base stations have no way to recognize such a situation and to replace the current base station with another one. Furthermore, the pre-configured slave base station cannot be re-configured as the master base station even though it can perceive the GPS signal stably and operate as a master base station.
A brief description is made of the IEEE 1588 standard.
The IEEE 1588 standard is a standard protocol defined for synchronization of clocks of distributed devices in a measurement and control system. The IEEE 1588 standard defines the protocol which categorizes devices into master and slave devices such that the clock of each slave device is synchronized with the clock of the master device. In an in-building environment, the synchronization can be acquired among the base stations by designating a base station capable of receiving a GPS signal as the master base station and transferring the GPS signal from the master base station to the slave base stations using the IEEE 1588 protocol.
A description is made of the operations of the master and slave base stations hereinafter. In the following, the description is directed towards the operations in view of the clock, but not base station management or the system in general. After being booted up, the master base station establishes an Assisted-GPS (A-GPS) connection to perform a GPS scan. In the GPS scan process, the master base station discovers the satellite to receive GPS information and thus, if an appropriate satellite is found, enters a locking state. If the signal meets a predetermined condition, the base station, as the master station, can transfer the clock information to the slave base stations.
Since it cannot operate as the master base station, the slave base station performs master (MST) scan right after initial boot-up without performing a GPS scanning process for discovering a satellite signal. The MST scan is a process to discover the base station as the IEEE 1588 master in the network, and the slave device performs clock synchronization with the master information acquired in the MST scan process. In case the slave has the record of the master information, the MST scan process can be omitted. After selecting the master base station, the slave base station exchanges clock information with the master base station to acquire clock synchronization.
As aforementioned, the IEEE 1588 protocol for conventional base station synchronization differentiates between the master and slave base stations such that the master base station acquires a clock signal from a satellite and transfers the clock signal to the slave base station using the IEEE 1588 protocol. In such an environment however, the roles of the base stations as the master and slave are determined at the installation stage. That is, the base station acting as the master pre-configures the information on the slave base stations to be managed, and each of the slave base stations pre-configures the information on the master base station. After installation, the slave base stations discover the master base station to establish network synchronization with the clock information provided by the found master base station. Although each slave base station can discover the master base station dynamically in the MST scan process, it is impossible to change the master base station, which has been already selected, with another base station and, as a consequence, this is equal to pre-configuration.
If the master base station fails to receive a signal from a GPS satellite, the master base station cannot provide the slave base stations with clock information which causes synchronization failure of the entire network, resulting in a service breakdown. Also, if there is a network disconnection problem even though the master base station has received the satellite signal successfully, the slave base stations cannot receive the clock information. Since each slave base station cannot change the current master base station for another base station, if the clock signal is not received from the current master base station, the slave base station cannot maintain the clock synchronization with the master base station.